1. Field of the Invention
The present invention relates to an image pickup lens for forming an image of an object on a solid-state imaging element such as a CCD sensor, a C-MOS sensor and the like used in a small-sized imaging device, and especially relates to an image pickup lens built into an imaging device mounted on a portable terminal such as a cellular phone or a smartphone, a PDA (Personal Digital Assistance), a game machine, an information terminal such as a personal computer, where downsizing and thinning are pursued.
2. Description of the Related Art
Recently, the market for portable terminals having imaging devices is increasingly expanding. Most portable terminals are equipped with a camera function, and currently, a majority of such camera function has a large number of pixels comparable to that of digital cameras. Along with the increasing demands for thinning of portable terminals from viewpoint of user-friendliness and design, demands for downsizing and thinning of the imaging devices built therein are also becoming severe. Further, regarding an image pickup lens mounted on a camera for taking a picture of oneself called an in-camera or a sub-camera in a portable terminal, a major type of the lens has an increased number of pixels exceeding 1 mega pixels, greater than the conventional VGA-class lens. The image pickup lens adopted in the imaging device using imaging elements having such increased number of pixels must have even higher resolution, be downsized and thinned, and have a small F value. At the same time, there is a strong demand for the lens to cope with a wider angle of field which enables the camera to take an image of an object in a wide area.
Generally, the image pickup lens answering to such trend of downsizing, thinning and enhanced performance is composed of multiple lenses. In the prior art, image pickup lenses having a two-lens configuration or a three-lens configuration have been adopted widely to realize a VGA class to one-megapixel-class lens, because they are advantageous from the viewpoint of size and cost. However, in order to cope with further downsizing and increase in the number of pixels, many image pickup lenses having a four-lens configuration capable of realizing a higher performance than the three-lens configuration have been proposed.
For example, Japanese Patent Laid-Open No. 2007-286153 (Patent Document 1) discloses an image pickup lens composed of, in order from an object side, an aperture stop, a first lens having a positive refractive power, a second lens having a negative refractive power with a concave surface facing an image side, a third lens having a positive refractive power, and a fourth lens having a negative refractive power with a concave surface facing the image side, wherein a value of a curvature radius of an image side surface of the second lens with respect to a focal length of the overall optical system is set to fall within an appropriate range with the aim to realize high performance.
Further, Japanese Patent Laid-Open No. 2008-046526 (Patent Document 2) discloses an image pickup lens composed of, in order from an object side, an aperture stop, a first lens having a positive refractive power, a second lens having a negative refractive power, a third lens having a positive refractive power, and a fourth lens having a negative refractive power with a concave surface facing the object side in which at least one side is formed as an aspheric surface, wherein a power of the first lens and the relationship of a curvature radius of an object side surface and an image side surface of the fourth lens are set to fall within an appropriate range with the aim to realize high performance.
Furthermore, Japanese Patent Laid-Open No. 2008-242180 (Patent Document 3) discloses an image pickup lens composed of, in order from an object side, an aperture stop, a first lens having a positive refractive power, a second lens having a negative refractive power, a third lens having a positive refractive power, and a fourth lens having a negative refractive power adopting a biconcave configuration and having at least one surface formed as an aspheric surface, wherein a ratio of a focal length of the overall optical system to a focal length of the first lens and to a focal length of the third lens is set to fall within an appropriate range with the aim to realize high performance.
Moreover, Japanese Patent Laid-Open No. 2009-014899 (Patent Document 4) discloses an image pickup lens composed of, in order from an object side, a stop, a first lens having a positive power with a biconvex shape, a second lens having a negative power with a meniscus shape and having a convex surface facing the object side, a third lens having a positive power with a meniscus shape and having a convex surface facing the image side, and a fourth lens having a negative power with a meniscus shape and having a convex surface facing the object side, wherein a relationship between a center thickness of the first lens and a focal length of the first lens and an Abbe number of the second lens and the third lens are set to fall within an appropriate range with the aim to realize high performance.
According to the image pickup lenses disclosed in Patent Documents 1, 2 and 3, a ratio (TTL/2IH) of a total track length (TTL) to a maximum image height (IH) is approximately 1.0, so that downsizing of the lens is comparatively realized. However, the half angle of field of the lens is 30 to 31°, which is insufficient for answering to the demand of wider angle of field. In addition, the F value of the lenses is between 2.88 and 3.29, which cannot be recognized as ensuring brightness sufficient for imaging elements having increased number of pixels. According to the image pickup lens taught in Patent Document 4, downsizing is also comparatively realized, but the F value thereof is 3.2, which means that sufficient brightness is not achieved. As described, it was not possible according to the prior art lenses to answer to the demands of downsizing, wider angle of field and small F value at the same time.